It is commonplace for digital communication devices to employ an architecture including an encoder, an interleaver and a modulator. The encoder receives data bits, convolves the data bits, and outputs encoded bits. The encoded bits are received by an interleaver, which re-sequences the encoded bits, and delivers the re-sequenced bits to a modulation system. The modulation system translates the encoded bits into symbols that are modulated upon one or more carrier frequencies for transmission to one or more receiving devices.
The ratio between the number of received data bits and the number of encoded bits generated by the encoder is termed the code rate, k/n. The number of stages of memory employed by an encoder is termed the constraint length, K. Any set of nK consecutive bits yielded by an encoder are correlated. For the sake of “spreading out” the impact of noise and interference during transmission, it is important not to carry correlated bits on the same transmission symbol. Therefore, if a transmission symbol is determined by a quantity of B bits, none of the B bits determining the symbol should be correlated. An interleaver may re-sequence the bits prior to their delivery to a modulation system, to ensure that such a condition is satisfied.
From the foregoing, it is evident that the level of required separation of encoded bits is proportional to the constraint length, K, of the encoder. As communication systems employ encoders exhibiting ever-greater constraint lengths, it is important to ensure that the ability to separate bits determining a transmission symbol is improved.
For the foregoing reason, it is desirable to employ an interleaving scheme that ensures that encoded bits are greatly and uniformly separated.